Short answer: Mechanically, it does nothing. It also does virtually nothing in a space fight because Blast extends to Short on the Personal-scale. It’s more about fortifications, mass troop formations, and troops formed around a vehicle (such as the clones we see walking beside an AT-TE at various points, such as the First Battle of Geonosis).
Short answer part two: I don’t suggest using overpressure to affect those inside.
Anti-vehicle weaponry works on basically 2x2 principles: Penetration/damage, and mechanical/personnel damage.
Example: you shoot an AP round at a tank. It penetrates the armor and kills the crew. You’ve effectively made the tank combat ineffective, even if it isn’t so damaged that it can’t be remanned (e.g. the gun still works and the engine still runs).
Alternatively, you shoot an AP round and it takes out the engine, but doesn’t do much to the crew. You’ve effectively reduced the tank’s combat effectiveness, even though the crew is still alive (and you still have to worry about the gun).
Solid shot works through penetration and spalling, killing the crew with shrapnel. Even if it doesn’t penetrate, it can create fracturing inside the vehicle, similar in concept to blowout if you were to snap a board in half.
HE works through creating a powerful shockwave, which can kill the crew inside through overpressure. Armor dramatically reduces this effect.
(That’s the nutshell. It’s more complicated than that.)
What you’re talking about in this case is basically an APHE round, which is designed to penetrate and then detonate, dealing damage by EITHER embedding into the armor and doing more structural damage than if the direction of the blast wave was perpendicular to the armor, OR by penetrating and detonating inside of the target, dealing explosive damage to exposed personnel and components (while venting atmosphere, if in vacuum).
I’m rambling again, sorry…
I have a houserule where if Breach is in excess of Armor, it penetrates the hull, creating a breach. I posted about it elsewhere on the forum and the details are in my space combat doc under “Optional Rules.”
Blast affecting those inside the vehicle is an interesting idea, but the implementation would be difficult. For example, if you simply apply Blast straight, then a concussion missile will instantly knock out any ship or vehicle small enough to be within the radius (the radius of Planetary-scale Blast is Short on the personal scale, whereas Personal-scale weapons, unless otherwise mentioned, have a radius of Engaged). And if you want to actually apply radii to the calculation, you have to not only know the size of the target but where the target was hit, and that brings up its own complications.
With a missile tube, you really aren’t going to be able to overpressure an Imperial shuttle, Sentinel or Lambda. Not only do they have shields and thick armor for spacefaring, but the troop compartment is insulated on both sides by external storage. The shockwave would be marginal by the time it got into the troop compartment. If memory of the mechanics serves me, the larger internal space also fights against you.
However, with an ITT, AT-ST, etc., you just might. If the ITT is locked up tight so the shockwave can’t well escape (if memory serves, soldiers would sometimes open hatches in heavily mined areas so that the blast waves would escape more easily… I could be mistaken), then overpressure in such a confined space is an option.
It is necessary to discuss the rules generally, though, since a rule that works in one situation may be hopelessly broken in another.
If you want to apply Blast damage to the passengers, I would ignore Breach, considering it “already applied” and instead reduce the damage by both the vehicle’s Armor and the individuals’ Soak.
If Breach equals or exceeds Armor, then I’d be willing to say that the missile goes in and detonates, killing everyone in the compartment. In my houserule, I make it 2 Advantage or a Triumph to penetrate, dealing more than external/structural damage.
In this situation, with these rules, a missile tube wouldn’t cut it. Missile tubes are really for soft targets and masses of infantry, not armored targets. While in theory intended to allow characters to tangle with vehicles, it really does not bridge the gap.
Worth noting in addition to this that “hits” on starfighters are likely proximity bursts more than direct hits, since a direct hit would obliterate a starfighter. Consider modern missile combat, which operates virtually identically to what I described.
I’m very rambley today, probably due for a reboot. I hope this is useful and not too incoherent.
